Underwater deposits of manganese bearing ores have been investigated for many years, but only recently have economic and environmental concerns encouraged the development of these materials as a viable alternative to conventional onshore mining. The underwater deposits variously referred to as marine nodules, ocean-floor nodules, manganese nodules, or maritime manganese ores, are distributed throughout the open oceans of the world and have recently been discovered on the floor of inland lakes such as Lake Michigan.
Although the distribution of marine nodules is broad, it is also quite uneven, with areas of concentration occurring both at great ocean depths and on the shallower continental shelf. The characteristics of the nodules vary from one deposit to another, showing considerable differences in size, shape, internal structure, and composition. In general, the nodules are primarily agglomerates of manganese and iron oxides, containing lesser proportions of compounds of copper, nickel and cobalt along with traces of some two dozen metals such as molybdenum, lead, barium, vanadium, chromium, and titanium, as well as various minerals plus alkali and alkaline earth metals. The metal content is generally dependent upon the region from which the nodules are obtained, and the economically important components may occur within broad ranges. For example, manganese content may range from 5 to 50 percent (dry weight), iron from 5 to 40 percent, copper from 0.03 to 2 percent, nickel from 0.1 to 2 percent, and cobalt from 0.01 to 3 percent.
This wide variation in metal values has complicated the refining of nodules and has led to the development of numerous processes for extracting the desirable materials. Straightforward physical methods for separating the metals have not been successful due to the extremely fine-grained nature of the primary manganese oxide and iron oxide constituents. Pyrometallurgical techniques likewise have not been notably successful, probably due to the formation of complex alloys that are quite difficult to separate. Consequently, the most effort has been directed to hydrometallurgical processes.
The chemical approaches to nodule benefication have centered on leaching, since this technique eliminates the need for drying the nodules to remove the 30-40 weight percent of moisture normally present. Several investigators have used a sulfuric acid leach in multistage autoclaves at 230.degree. C. and 500 psig. The acid readily dissolves copper, nickel and cobalt under these conditions, and these metals are recovered by liquid ion exchange and electrowinning. Large quantities of acid are required, probably due to the presence of basic materials trapped in the nodule, and significant quantities of iron and manganese are also dissolved into the leach liquor. Most of the manganese in the nodules is discarded in the tailings from this process.
Other workers have based a leaching process on the use of hydrogen chloride to convert the metals to soluble chlorides. Ferric chloride is removed from solution by solvent extraction, then decomposed to Fe.sub.2 O.sub.3 and HCl. Copper, nickel, and cobalt are coprecipitated by cementation with manganese metal, then are separated by liquid ion exchange and recovered by electrowinning. The manganese chloride is processed by displacement with aluminum metal to recover manganese metal. Thus expensive processing steps and reagents are necessary to deal with the iron and manganese extracted, which places the recovered manganese at a cost disadvantage.
Another method involves crushing, drying, and roasting of the nodules in a reducing atmosphere, followed by leaching with an ammonia-ammonium carbonate solution. Countercurrent decantation washes the solubilized metals, primarily copper, nickel and cobalt, from the gangue. Liquid ion exchange removes the metals from the leach liquor and separates them for recovery by electrowinning or precipitation. Manganese is discarded with the gangue.
It is apparent in all of the conventional processing schemes that manganese, the principal component of marine nodules, is a liability. Either it is discarded with the process tailings or it is recovered at considerable expense, resulting in an economic disadvantage to the entire process. Conventional techniques have not provided a simple, direct method for selectively separating iron and manganese from the nodules and from each other without high processing costs, while at the same time efficiently recovering other desirable metal values such as copper, nickel and cobalt from the nodules.